Advanced Custom Fields bi-directional relationship

acf_bidirectional_relationship.php

<?php

/**
 * Set bi-directional post type relationship
 */

add_filter( 'acf/update_value/type=relationship', 'themeslug_acf_bidirectional_relationship', 10, 3 );

function themeslug_acf_bidirectional_relationship( $value, $post_id, $field ) {

	$allowed_field_names = array( 'car_model', 'car_make' );
	$field_name = $field['name'];
	$remote_name = ( 'car_model' === $field_name ) ? 'car_make' : 'car_model';

	if ( ! in_array( $field_name, $allowed_field_names ) ) return $value;

	remove_filter( 'acf/update_value/type=relationship', 'themeslug_acf_bidirectional_relationship', 10, 3 );

	// add current post_id to remote value
	if ( ! empty( $value ) && is_array( $value ) ) {
		foreach ( $value as $remote_id ) {
			$remote_value = get_field( $remote_name, $remote_id, false );
			$remote_value = ( ! empty( $remote_value ) ) ? $remote_value : array();

			if ( ! in_array( $post_id, $remote_value ) ) {
				$remote_value[] = $post_id;
				update_field( $remote_name, $remote_value, $remote_id );
			}
		}
	}

	$old_value = get_field( $field_name, $post_id, false );

	if ( ! empty( $old_value ) && is_array( $old_value ) ) {
		foreach ( $old_value as $remote_id ) {
			// if remote_id from old value is not found in the current new value array, we remove it
			$value = ( ! empty( $value ) ) ? $value : array();

			if ( ! in_array( $remote_id, $value ) ) {
				$remote_value = get_field( $remote_name, $remote_id, false );

				if ( ! empty( $remote_value ) && is_array( $remote_value ) ) {
					$pos = array_search( $post_id, $remote_value );
					unset( $remote_value[ $pos ] );
					update_field( $remote_name, $remote_value, $remote_id );
				}
			}
		}
	}

	add_filter( 'acf/update_value/type=relationship', 'themeslug_acf_bidirectional_relationship', 10, 3 );

	return $value;
}